Merkel cell carcinoma (MCC) is a highly aggressive form of skin cancer that typically affects older and/or immunosuppressed individuals. Although MCC is relatively rare, its incidence in the U.S. has increased dramatically in the past decade. In 2008, DNA from a previously-unidentified polyomavirus was found to be present in a substantial majority of MCC tumors. The result suggests the possibility that the virus may be an etiologic cause of MCC and perhaps other forms of cancer. In FY08, work in the Tumor Virus Molecular Biology Section was focused on the biology of the human papillomavirus (HPV) virion. In FY09 we succeeded in applying a variety of tools originally developed for HPV research to the study of the newly-discovered Merkel cell polyomavirus (MCV). As a result, our Section is now well situated to rapidly answer several important questions about MCV biology. MCV-based reporter vectors are a primary technology for this project. Production of the MCV vectors relies on intracellular expression of the MCV capsid proteins, which are capable of packaging reporter plasmids of interest in the milieu of the cell nucleus. The MCV reporter vectors are capable of delivering the packaged reporter plasmid to cultured cells, likely via mechanisms that resemble the infectious entry pathway of authentic MCV. In FY09, we found that the infectivity of MCV vectors can be inhibited by MCV-specific antibodies. This allowed us to develop a high-throughput assay for quantitative analysis of antibody responses elicited by natural infection with MCV. We found that a great majority of adults display MCV-specific antibody responses, suggesting that infection is very common. Although infection appears to be common, we found that the magnitude of normal adults'antibody responses to MCV was typically much lower than the MCV-specific antibody responses observed in MCC patients. The result suggests a model in which MCC patients have typically experienced an unusually immunogenic form of MCV infection. Extremely strong antibody responses against MCV might thus serve as a biomarker for MCV-associated diseases. In FY10, we intend to extend our serological studies to investigate possible links between MCV and additional cancers, such as chronic lymphocytic leukemia. In the longer term, we will pursue investigation of the infectious entry biology of MCV. We have completed an initial effort in this area, in which we screened the "NCI-60" panel of tumor cell lines for susceptibility to transduction with MCV reporter vectors. The screen revealed an interesting tissue-specific effects that may help elucidate the natural site of MCV infection.